1. Field of the Invention
The present invention involves clutch drivers. More particularly, the present invention involves drivers for controlling a solenoid over a wide range of temperatures and voltages.
2. Discussion of the Related Art
Solenoids are commonly used for engaging and disengaging servo clutches. FIG. 1 illustrates a related art mechanism 100 involving a servo clutch using a solenoid. Mechanism 100 includes a motor 105, an input drive shaft 117 connected to the motor 105, and a clutch 115 that includes a solenoid 120. The clutch includes a drive plate 116, which is connected to input drive shaft 117, and a load plate 122, which is connected to load drive shaft 110. The solenoid 120 includes a coil 121 that is connected to a power supply 125.
Adverse temperature conditions affect the function of related art mechanism 100. For example, under high temperatures, the resistance of coil 121 increases, which reduces the current through coil 121. The force that keeps clutch 115 engaged, referred to as a closure force, is equal to the current through coil 121 multiplied by the number of turns of wire within coil 121. The closure force relates to the torque handling capacity of clutch 115. As the temperature of coil 121 increases, the current may decrease to the point to where the closure force is insufficient to keep drive plate 116 and load plate 122 mechanically connected, and clutch 115 may slip or disengage. Under these conditions, the torque handling capacity is diminished and clutch 115 may slip at lesser loads than under nominal conditions.
A further problem associated with related art mechanism 100 is that the closure force provided by solenoid 120 is subject to line voltage fluctuations from the power supply 125. Reduction in line voltage reduces the current in coil 121, which may cause clutch 115 to disengage due to subsequent loss of closure force.
In order to maintain closure force at high temperatures and/or reduced line voltage, coil 121 may be designed for a worst case scenario with respect to high temperature and/or low line voltage. In doing so, coil 121 may be designed to nominally have sufficiently low resistance so that, if the coil resistance increases with temperature, and/or the line voltage decreases, the resistance does not increase to the point where the current in the coil is insufficient to maintain closure force. Accordingly, a coil 121 designed to maintain closure force under a worst case scenario will consume excess power under nominal conditions. The heat generated by coil 121, due to excess power consumption, may adversely affect its reliability. One possible effect of heat due to excess power consumption is the breakdown of insulation between turns of wire in the coil 121, which may lead to failure of coil 121. Further, heat generated by coil 121 may be transferred to other electronic components, adversely affecting their reliability.
Accordingly, there is a need for a servo clutch driver that can operate under a wide temperature range and maintain closure force in the presence of voltage fluctuations, while minimizing power requirements.